Merge bitcoin/bitcoin#36113: psbt: fix rendering for invalid long sighash type field

### src/core_io.cpp
@@ -338,9 +338,13 @@ const std::map<unsigned char, std::string> mapSigHashTypes = {
     {static_cast<unsigned char>(SIGHASH_SINGLE|SIGHASH_ANYONECANPAY), std::string("SINGLE|ANYONECANPAY")},
 };
 
-std::string SighashToStr(unsigned char sighash_type)
+std::string SighashToStr(int32_t sighash_type)
 {
-    const auto& it = mapSigHashTypes.find(sighash_type);
+    // Signatures encode the sighash type in a single byte, but the PSBT field
+    // for it is a 32 bit unsigned integer in BIP 174 (signed in PSBTInput)
+    if (sighash_type < 0 || sighash_type > 0xff) return "";
+    const uint8_t sighash_byte(sighash_type);
+    const auto& it = mapSigHashTypes.find(sighash_byte);
     if (it == mapSigHashTypes.end()) return "";
     return it->second;
 }

### src/core_io.h
@@ -8,6 +8,7 @@
 #include <consensus/amount.h>
 #include <util/result.h>
 
+#include <cstdint>
 #include <functional>
 #include <string>
 
@@ -42,7 +43,7 @@ bool DecodeHexBlockHeader(CBlockHeader&, const std::string& hex_header);
 UniValue ValueFromAmount(CAmount amount);
 std::string FormatScript(const CScript& script);
 std::string EncodeHexTx(const CTransaction& tx);
-std::string SighashToStr(unsigned char sighash_type);
+std::string SighashToStr(int32_t sighash_type);
 void ScriptToUniv(const CScript& script, UniValue& out, bool include_hex = true, bool include_address = false, const SigningProvider* provider = nullptr);
 void TxToUniv(const CTransaction& tx, const uint256& block_hash, UniValue& entry, bool include_hex = true, const CTxUndo* txundo = nullptr, TxVerbosity verbosity = TxVerbosity::SHOW_DETAILS, std::function<bool(const CTxOut&)> is_change_func = {});
 

### src/rpc/rawtransaction.cpp
@@ -1224,7 +1224,7 @@ static RPCMethod decodepsbt()
 
         // Sighash
         if (input.sighash_type != std::nullopt) {
-            in.pushKV("sighash", SighashToStr((unsigned char)*input.sighash_type));
+            in.pushKV("sighash", SighashToStr(*input.sighash_type));
         }
 
         // Redeem script and witness script

### test/functional/rpc_psbt.py
@@ -557,6 +557,14 @@ def test_sighash_adding(self):
 
         wallet.unloadwallet()
 
+    def test_decodepsbt_long_sighash_type(self):
+        self.log.info("Test that decodepsbt rejects invalid trailing bytes in the sighash type field")
+        node = self.nodes[0]
+        psbt = PSBT.from_base64(node.createpsbt([{"txid": "00" * 32, "vout": 0}], [{"data": "00"}]))
+        # The first byte of this sighash type is ALL, but the type itself is not
+        psbt.i[0].map[PSBT_IN_SIGHASH_TYPE] = (0x101).to_bytes(4, "little")
+        assert_equal(node.decodepsbt(psbt.to_base64())["inputs"][0]["sighash"], "")
+
     def assert_change_type(self, psbtx, expected_type):
         """Assert that the given PSBT has a change output with the given type."""
 
@@ -1619,6 +1627,7 @@ def global_xpub_key(extended_pubkey):
         if not self.options.usecli:
             self.test_sighash_mismatch()
         self.test_sighash_adding()
+        self.test_decodepsbt_long_sighash_type()
         self.test_psbt_named_parameter_handling()
         self.test_psbt_roundtrip()
         self.test_psbt_version()